Process for the preparation of an organic acid ester of cellulose in fibrous form



Patented Apr. 6, 1937 UNITED, STATES/ PROCESS FOR THE PREPARATION OF AN ORGANIC ACID ESTER OF CELLULOSE IN FIBROUS FORM Frederick J. Hopkinson and Charles R. Fordyce,

Rochester, N. Y., assignors, by mesne assignments, to Eastman Kodak Company, Jersey City, N. J a corporation of New Jersey No Drawing. Application July 28, 1933, Serial No. 682,726

4 Claims. (Cl. 260101) The present invention relates to the preparation of organic acid esters of cellulose, such as cellulose acetate or cellulose acetate propionate, in fibrous form by means of a cyclic process.

5 The two most common methods of making organic acid esters of cellulose at the present time are (1) the dope method, in which the ester formed dissolves in the esterification bath, and (2) the fibrous method, in which the esterification bath is non-solvent of the ester formed therein so that it exhibits the form of the original cellulose at the completion of the esterification.

The dope method of preparing cellulose esters is probably that most commonly employed at the present time, however, in that method, due to the viscosity of the syrupy mixture or dope which forms and the speed with which the esterification proceeds, control of the temperature of the reaction mixture is dificult. This disadvantage may be alleviated if a large amount of bath is employed, however, the use of a large amount of bath or mixture adds to the quantity of material which must be recovered from a dope esterification process. Due to the particularity which is necessary in a dope esterification process, there is a tendency for the qualities of the successive esters produced to vary from one esterification to the next, even though the proportions of ingredients remain the same, and extreme care is necessary in this type of process to minimize this tendency.

It is necessary at the completion of a dope" method of preparing a cellulose ester and after the desired solubilities have been imparted thereto, to separate the ester from the spent esterification bath usually by mixing with the batha quantity of water sufficient to render it non-solvent of the ester formed; the latter then precipitates out, The quantity. of water necessary for this separation is suflicient to dilute the acid therein considerably and to convert any acid anhydride which may be present to its corresponding acid. Thus there remains a large quantity of a dilute aqueous solution of organic acid which, in the interests of economy, is usually reclaimed by separating the water therefrom, As was pointed out above, the larger the bath employed the greater the amount of material, usually organic acid, which must be recovered. The necessity of disposing of this great bulk of aqueous 50 organic acid adds materially to the cost of preparing an organic acid ester of cellulose in dope form.

The fibrous processes which have been proposed up to the present time have not found a 55 great deal of use commercially due probably to disadvantages of one sort or another which have heretofore confronted the use of those proposed processes.

In those processes, as is also true indope processes, due to the lack of fluidity of the mass, there is danger of uneven esterification caused by excessive local action; likewise the safe temperatures of esterification may be exceeded, all of which, contribute toward the formation of a degraded product. Although we are not aware of any disclosure in the prior art of proportions of ingredients which would result in a soupy or fluid mass when mixed with the cotton to be esterified, and even though the use of such a large amount of bath would avoid the local action and overheating met with in the ordinary fibrous esterification baths, the use of such large proportions of ingredients in an ordinary fibrous esterification process would render the cost-thereof so great, due to the expense involved in recovery of the spent esterification liquid, that it would prob- M ably be less practical than the use of a dope'esterification process.

One object of our invention. is to provide a process for the preparation of organic acid esters of cellulose in fibrous form in which the action on the cellulose is substantially uniform and which gives a product of good quality. Another object of our invention is to provide a process for the preparation of organic acid esters of cellulose in fibrous form in which the cost thereof is reduced to a minimum. A further object of our invention is to provide a process for esterifying cellulose in which only a small portion of the spent esterifying bath need be reclaimed, thus conserving to a large extent desirable an hydride values in the spent liquors. A still further object of our invention is to make possible the reuse of a large proportion of the spent liquor from a fibrous esterification bath. Other objects will appear herein.

We have discovered a cyclic process of preparing organic acid esters of cellulose in which the cellulose is esterified in a bath present in sufficient proportion with reference to the cellulose to render the entire mass fluid or soupy. We

have found that the spent esterification liquid from one esterification, with modification depend.- ing on the decrease of organic acid anhydride and increase of organic acid in the bath, may be employed for a subsequent esterification. We

have found that by use of alarge amount of liquid in a. fibrous esterification, the difliculty of con-. trolling temperature is overcome and a uniform,

'stable product is obtained, and by reuse of the spent esterification liquor the cost of the process is considerably reduced. In addition, the soupy reaction mixture may be readily handled such as by pumping of the like, so that it may be easily transferred from one vessel to another or to any other apparatus such as to a filter press or centrifuge after the completion of the reaction.

Our invention broadly comprises reacting upon cellulose with an esterification bath in which the amounts of the various ingredients are sufilcient to assure a fluid mass when the cellulose is added thereto, and then employing a major portion of the spent esterification liquid after its fortification for a subsequent esterification, thus forming a cyclic process for the preparation of cellulose esters. spent esterifying liquid which may be reused depends upon the amounts of the respective acids formed inthe esterification process, as will be exemplified hereinafter.

The present invention eliminates the danger of degradation of the cellulose due to the fact that temperature increases are only gradual and to the fact that the reaction proceeds slowly. The gradual rise in temperature may be attributed principally to the amount of liquid which is present in proportion to the cellulose, inasmuch as only a certain amount of heat can be produced when a given amount of cellulose is esterifled to form a particular ester. For instance, when 162 gms. of cellulose are acetylated, the heat given off is about 40,000 calories. As the amount of liquid is large the heat produced in the reaction is absorbed by a much larger mass than is usually thecase, so that the change of temperature is less than would be the case in an esterification process carried out in accordance with the prior art. Due to the fluidity of the mass, the heat is also carried away more rapidly than formerly. If desired, although unnecessary, the mass may be run through cooling means to assure more rapid dissipation of heat.

The following examples illustrate the application of our invention to the preparation of cellulose esters:

' Example I Five hundred pounds of cellulose are esterified with a mixture of 2000 lbs. of acetic anhydride, 1625 lbs. of 95% propionic anhydride, 2200 lbs. of glacial acetic acid, 720 lbs. of propionic acid, 1576 lbs. of trichlorethylene, 3424 lbs. of .Stoddard solvent and 37 lbs. of mixed catalyst (1 part by volume of 95% sulfuric acid to 3 parts by volume of 95% phosphoric acid). Stoddard solvent is a commercially well known petroleum hydrocarbon, the specifications of which are given in the Bureau of Standards bulletin CS3-28.

The foregoing esterification is accomplished by first pretreatlng the cellulose with a mixture comprising 960 lbs. acetic anhydride (95%) 1180 lbs. propionic anhydride (95%), 2200 lbs. glacial acetic acid, 670 lbs. 100% propionic acid, 2790 lbs. of Stoddard solvent, and 1282 lbs. of trichlorethylene, for 16 hrs. at 25 C. following which a mixture of 1040 lbs. of acetic anhydride (95%), 445 lbs. of propionic anhydride (95%), 58 lbs. of propionic acid (100%), 634 lbs. of Stoddard solvent, 294 lbs. of trichlorethylene, and 37 lbs. of mixed catalyst is added thereto and the whole maintained at 40 C. over a period of 24 hours or until the esterification is complete.

It will be noted from the table hereinafter that the pretreatment mixture in this original esterification was'calculated to correspond to the re- The proportion of the use liquor, and in this calculation a prediction was made, and later confirmed, as to the composition of the resulting ester. In order to determine the percentage of the spent esterification liquid which may be reused in a second esterification of the same size and proportions, it is necessary to determine approximately the amount of each acid formed. This may be done either by determining the amount of each anhydride remaining or preferably by determining approximately the acetyl and propionyl content of the ester formed.

The product of the esterification is cellulose acetate propionate and when analyzed by the method described hereinafter will be found to possess approximately 86 mol. per cent of acetyl and 14 mol. per cent of propionyl. The yield is assumed to be and the composition of the spent esterification liquid may be calculated. With the yield assumed-as 180%, the final product will weigh 900 lbs. The ester will contain 36.7% by weight of acetyl and 7.8% by weight of propionyl groups. The weight of acetic anhydride (100%), consumed in the reaction is calculated as follows:

900 (wt. of ester) .367 X The weight of propionic anhydride (100%) used up is calculated as follows:

In the present example as 95% acid anhydrides are employed it is necessary that the anhydrides be converted to 100% before subtracting the amounts of anhydrides used up and then that figure be converted back to 95% to ascertain the amount of 95% anhydride present. The calculations may be made by means of a mechanical calculating means which is sufliciently accurate for ascertaining the amount of each acid anhydride present in the spent esterification liquid. The amount of each acid anhydride consumed is thus arrived at from the analysis of the ester, by calculating the weight of acetyl and propionyl groups needed to completely esterify 500 lbs. of cellulose in order to obtain a product of the analysis given.

The amount of acid present in the spent liquor may be calculated by adding to the original amounts of the respective acids employed in the reaction mixture, the amount of the additional acids formed by the using up of the anhydrides. The amounts of non-solvent in the spent liquid may be considered for all practical purposes th same as in the original bath.

The catalyst employed in this example, however, may not be considered constant, due to removal of part thereof by the product, as combined sulfur containing radicals.

Alternatively, the catalyst employed in the original esterification may be' removed from the spent esterification liquor by percolating it through an adsorbing material such as activated charcoal, this catalyst removal process being the invention of C. J. Malm and G. F. Nadeau and is described and claimed in their application Serial No. 682,724 of even date.

The amount of anhydride employed in removing the moisture from the cellulose, need not be taken into consideration, however, if the precise carrying out of this invention should be desired it would be necessary that all the factors which affect the composition or the amount of the esterification mixture be taken into consideration.

To summarize Example I, the composition of the original esterification bath is:

The use of a mixture of Stoddard solvent andtrichlorethylene as the non-solvent in a ilbrous esterification process is not our invention but is the invention of C. J. Malm and C. L. Fletcher and is disclosed and claimed in their application Serial No. 671,901. It was calculated, as explained above, that the esterification bath when spent will result in a composition approximately as 25 follows:

(28.00 lbs. HaPO4) As the amount of glacial acetic acid in the spent liquor is about A. more than that which was 40 employed in the original esterification bath when all chemicals are calculated as of 100% strength,

only 81.5% of the spent esterification bath may be employed to make up the second esteriflcation bath having the same formula as the first. This 45 81.5% portion will comprise:

Pounds Acetic anhydride (95%)- 960 Propionic anhydride (95%) 1180 Glacial acetic acid 2200 Propionic acid (100%.. 6'70 Stoddard solvent 2790 Trichlorethylene 1282 Mixed catalyst 28.3 5lbs.mSO4) (22 lbs. I'1iP04) In order to prepare the second and subsequent esterification baths of a composition similar to: the original it becomes necessary to add to the 0 81.5% of the spent liquor which will be termed the reuse liquor, the following ingredients in the amounts indicated:

Pounds 65 Acetic anhydride (95%) 1040 Propionic anhydride Propionic acid (100%)--- 58 Stoddard solvent 634 70 Trichlorethylene 294 Catalyst 8.7 (3.5 lbs. H2804) (5.2 lbs. HaPOO The second and subsequent esterifications are 75 carried out by pretreating 500 lbs of cellulose with the reuse liquor (81.5% of the spent liquor) for 16 hrs. at 25 0.; the make-up or fortiiying mixture is then added to the mass and the whole maintained at 40 C. for 24 hrs. or until the esterification is complete. The product in each esterification obtained will be similar in composition and properties to that obtained in the original esterification. The 18.5% of the spent liquor may be processed to separate the acids, anhydrides and non-solvents therefrom, which, when separated, may be employed in subsequent esterifications of the same series or in other esteriflcation processes.

Example II In this example, a less soupy formula is employed to esterify the cellulose but the conditions are maintained substantially the same as before. The amount of reuse in the second and subsequent repetitions of the cycle is estimated at about 75% and a pretreatment bath of the estimated composition of the reuse liquid is em ployed. 500 lbs. of refined cotton linters are allowed to stand in the pretreatment bath for 16 hrs. at 25 C. (The original pretreatment bath may differ somewhat from the actual reuse composition without materially aifecting the process). The chemicals necessary to bring the pretreatment bath up to the composition of the esterifying bath are then added and the mass maintained at a temperature of 40 C. for 41 hrs. or until the esterification is complete. The compositions of the original esterifying bath and of the reuse from which the composition of the complete esterifying bath and of the pretreatment bath may be determined,-are listed below. The ester formed is cellulose acetate propionate and will have an analysis of approximately 84.5 mol. per cent of acetyl and 15.5 mol. per cent of propionyl or 36.8% acetyl and 8.9% propionyl by weight. Due to the amount of acid calculated to be present in the spent liquor, the amount of reuse thereof in this example is 75.6%. The composition of the original esterification bath, the spent liquor, the reuse liquor, and the fortifying mixture (with which to make up the reuse liquor to the composition of the original esterifying bath) in order to carry out a second esterification of 500 lbs. of cellulose, all in lbs., is as follows:-

In this process the same proportions of ingredients are employed as in Example II but with the difference that the cellulose is pretreated with acetic acid only, prior to its esterification. 500 lbs. of cotton linters are allowed to stand in 2100 lbs. of glacial acetic acid fonalfi hours at room temperature (25 C.) and the] mass then centrifuged so that the'whole weighs '750 lbs. or, in other words, 1850 lbs. of acetic acid are removed therefrom. The mass is then washed with 4750 lbs. of Stoddard solvent and centrifuged until it retains 500 lbs. of liquid (80 lbs. acetic acid and 420 lbs. Stoddard solvent).

The whole is then combined with a mixture consisting of Pounds Acetic anhydride 1340 Propionic anhydride 1010 Glacial acetic acid 1356 Propionic acid (100%) 690 Stoddard solvent 3580 Trichlorethylene 600 Sulfuric acid (conc.) 7.5

and the mass maintained at 40 C. until the esterification is complete (about 40 hrs.)

The ester formed is cellulose acetate propionate and will show upon analysis an acyl content of approximately 86.5 mol. per cent of acetyl and 13.5 mol. per cent of propionyl which corresponds to 37 .1% of acetyl and 7.8% propionyl by weight based on the completely esterified product. Due to the fact that some acetic acid is already present in the cotton when it is added to the esterification bath in the process of this example, the percentage of re-use of the spent esterification liquor may be expected to be lower than where the cellu-. lose is not pretreated with a mixture containing acetic acid. In the present example the reuse of the spent esterification liquor is 71%. The compositions of the original bath, the spent liquor, the reuse liquor, and the fortlfying mixture, all in lbs. for reacting upon 500 lbs. of cellulose are as follows:

Esterify Reuse ing Spent (71% of g bath spent) p Acetic anhydride (l%) 1340 540 383 957 Propiouic anhydride (100%)" 1010 850 604 406 Acetic acid (glad) 1436 1906 1356 None Propionic acid (100%) 690 781 555 135 Stoddard solvent- 4000 4000 2840 740 Trichlorethylene 600 600 426 174 Sulfuric acid 7. 5 5. 25 3. 75 3. 75

The same procedure is employed with the second and subsequent 500 lb. batches of cotton linters except that the esterifying bath into which the pretreated cellulose is placed, instead of beingcomposed of fresh ingredients is made up from 71% of the spent esterification liquor together with the fortifying ingredients as exemplified in the above table. Products of substantially the same composition as that obtained in the original esterification will result.

I Example I In this process the same proportions of ingredients are employed as in Example II, however, the cellulose is pretreated with a mixture of acetic acid and Stoddard solvent. The pretreatment of cellulose with a mixture of a fatty acid and a non-solvent to prepare it for esterification is not our joint invention but is the invention of E. C. Yackel and W. O. Kenyon and is disclosed and claimed in their application I Serial No. 671,898 filed M88 19, 1933.

500 lbs. of refined cotton linters are allowed to stand in a mixture of 380 lbs. of acetic acid and 3620 lbs. of Stoddard solvent for 16 hrs. at 40 C. and themass is then centrifuged until 250 lbs. of liquid remains on the cotton. This 250 lbs. of liquid consists of 111 lbs. of acetic acid and 139 lbs. of Stoddard solvent. It will be observed that these components are not in the same proportion as that of the pretreatment liquor, due to differential retention of acetic acid by the cotton. The cellulose containing the 250 lbs. of liquid is placed in the esterifying mixture consisting of 1340 lbs. of acetic anhydride, 1010 lbs. of propionic anhydride, 1325 lbs. of glacial acetic acid, 690 lbs. of propionic acid, 3861 lbs. of Stoddard solvent, 600 lbs. of trichlorethylene and 3.7 lbs. of sulfuric acid. The whole is maintained at 40 C. for 72 hrs. to completely esterify the cellulose. The ester in fibrous form is separated from the reaction mixture and the spent esterification liquor stored for further use in esterification processes of a like nature. The product is cellulose acetate propionate and on analysis will be found to have an 'acyl content approximately 83.5 mol. per cent of acetyl and 16.5 mol. per cent of propionyl which corresponds to a content of 36.3% by weight of acetyl and 9.5% by weight of propionyl based on the total ester.

As was the case in the previous example, the percentage of reuse of the spent esterification liquor in this example will be limited by the fact that the acetic acid in the esterification bath is supplemented by that retained by the pretreated cellulose. From the amount of acetic acid in the spent liquor and the amount thereof which is necessary to reconstruct a composition of the original formula it .may be seen that the spent liquor will allow of 70% reuse.

The compositions of the original bath including the liquid retained'by the cotton from its pretreatment, the spent liquor, the reuse liquor,

and the fortifying mixture, all in lbs. are as follows:

Esterify- Reuse ing Spent 70% of g bath spent) P Acetic anhydride (l00%) 1340 565 395 945 Propionlc anhydride (100%) 1010 815 570 440 Acetic acid (glacial) 1436 1892 1325 None Propionic acid (100%) 690 801 561 129 Stoddard solvenL. 4000 4000 2800 106i Trichlorethylene 600 600 420 180 Sulfuric acid 3. 7 1.7 1.2 2. 5

-It isto be noted that here as in the previous example, the sum of the reuse and the make-up differs from the amount used in the esterifying bath by the amount of acetic acid and Stoddard solvent retained by the cellulose from the pretreating liquor;

Subsequent 500 lb. batches of cellulose may be esterified cyclically, the conditions being the same as in the original process except that esterification baths made up from' the spent liq uors from previous esterifications are employed. Products of substantially the same composition are obtained.

Example V The present invention may be carried out without analyzing the resulting ester especially in the case where the individual operator has had some experience with the preparation of cellulose acetate-propionate or cellulose acetate, so that the amount of acid anhydrides used up in the process may be estimated without the necessity of analyzing the ester formed. In the present example the amounts of the respective anhydrides used up in the esterification are calculated from an estimate of the composition of the ester formed;

500 lbs. of refined cotton linters are reacted upon with an esterification mixture consisting of Glacial acetic acid 1400 lbs. Propionic acid 1000 lbs. Acetic anhydride 1000 lbs. Propionic anhydride 500 lbs. N-butyl ether 3000 lbs. Sulfuric-phosphoric acid 6750 cc. Or sulfuric-phosphoric acid 27.5 lbs.

(consisting of 3 volumes of phosphoric acid and 1 volume of sulfuric acid) at a temperature of 90 F. and the temperature of the mass gradually raised to 130 F. over a period of 4 hours at which temperature it is maintained for 36 hrs.

The fibrous product is separated from the reaction mixture by centrifuging. The product is cellulose acetate propionate and is soluble in ethylene dichloride and it is assumed from the proportions of the esterifying chemicals that the propionyl groups contribute about of the total acyl groups present in the ester. Based on this assumption, the composition of the spent esterifying liquid is calculated to be:

'30 Pounds Acetic i 1752 Propionic acid 1234 Acetic anhydride 400 Propionic anhydride -90 N-butyl ether 3000 Catalyst 27.5

Upon consideration of the amounts of the acids in the spent bath and the amounts needed it will be seen that .1000/1234 (propionic acid) 40 or 1400/1752 (acetic acid) which is approximately 80% of the spent liquor, may be reused. The reuse liquor will have a composition of approximately 1 Pounds Acetic acid 1400 Propionic acid 1000 Acetic anhydride 320 Propionic anhydride 72 N-butyl ether 2400 Catalyst 22 In order to make this composition up to a composition corresponding to that of the orig inal esterification bath, it is necessary .to add thereto the following:

Pounds Acetic anhydride 680 Propionic anhydride 428 N-butyl ether 600 Catalyst 5.5

These figures are given merely byway of example, and do not take into account the sulfuric acid removed by the product.

. A second 500 lbs. of cellulose may be added to the esterification bath thus made up to correspond to the original and the esterification carried out in the same manner and at the same temperatures as in the first esterification.

Example VI 30 lbs. of refined cotton linters are allowed to stand in 240 lbs. of a mixture of 23 lbs. of glacial acetic acid and 217 lbs. of Stoddard solvent for 16 hrs. at 40 C. and the mass is then centrifuged until 15 lbs. of liquid (6.7 lbs. of acetic acid and 8.3 lbs. of Stoddard solvent) remains on the cotton. The cellulose containing the 15 lbs. of liquid is then placed in an esterifying mixture consisting of 102.6 lbs. of 100% aceticanhydride, 162.9 lbs. of glacial acetic acid, .55 lb. of sulfuric acid, 131 lbs. of trichlorethylene and 135.7 lbs. of Stoddard solvent and the whole is maintained at 40 C. for approximately 36 hours or until the cellulose triacetate (44.8% acetyl) is formed. From the amount of acetic acid present in the spent liquor and the amount which is necessary to reconstruct a composition of the original formula, it may be seen that the spent liquor will allow of reuse.

The compositions of the original bath (excluding the liquid retained by the cotton from its pretreatment), the spent liquor, the reuse liquor and the fortifying mixture all in lbs. are

as follows:

. Re-use Estenly- S Makemg bath Spent) up Acetic anhydride 100%... 102. 6 45. 6 36. 5 66. 1 Glacial acetic acid 162. 9 203. 1 162. 9 'lrichlorethylene 131 131 105 26 Stoddard solvent 135. 7 144 115. 5 28. 5 Sulfuric acid 0. 55 0. 25 0. 2 0.35

The reuse liquor may be made up to the composition of the original esterifying bath and a second batch of cellulose after pretreatment may then be acetylated therewith to form a cellulose triacetate.

Example VII 40 C. for 31 hrs. to completely esteri'fy the cellulose. The product is cellulose acetate butyrate and on analysis will be found to have an apparent acetyl content of 43.0% and an acyl content of 87.8 mol. per cent of acetyl and 12.2 mol. per cent of butyryl, which corresponds to a content of 37.75% by weight of acetyl and 8.67% by weight of butyryl. The compositions of the various mixtures in this instance will be as follows:

Li uid tained e i: s t ($13 Mak q re om pres pen 0 atreat mixture liquor spent) up liquor Acetic anhydride 1083 277 214 869 Butyi'ic anhydride (100%) 1238 1064 820 418 Glacial acetic acid 111 1972 2557 1972 Butyric acid (101%) 833 930 716 117 Stoddard solvent 139 2494 2633 2030 Suliul'ic acid (95%)-..- 6. 0 4. 0 3. 1 2. 9

The revivifled or restored spent esterification liquor may be employed for the subsequent esteriflcation bath in each succeeding esterification.

In any of the above examples where 100% acid anhydrides are employed it is to be noted that the percentage of reuse is greater than where for instance 95% anhydrides are employed, as some fatty acid is introduced by this anhydride and this additional fatty acid may be compensated for by reducing the percentage of reuse. If the anhydride of the fatty acid on which the percentage of reuse is based is of lesser strength than 100%, such as one containing 85%, the percentage of reuse must necessarily be further decreased to compensate for this further dilution.

It is of course to be understood that the present invention is not confined only to cyclic processes in which the second or subsequent esteriflcation baths are of the same composition as the first. For instance a cyclic process in accordance with 10 the present invention may be performed in which only alternate esterlfication bathsmight correspond with each other. A series of esterifications might be carried out in accordance with our invention in which every third or even fourth bath is similar, however, cyclic processes in which none of the esterificatlon baths are similar but in which at least a portion of the spent esterification liquor is reused in making up the esterifying mixture will also be within the scope of our invention.

A series of esterifications in accordance with the present invention, providing each successive esterification bath is made up to the composition of the original, will extend over any number of successive esterifications and the results obtained in each instance will show excellent agreement.

For example the ester prepared in the fifth successive cycle in which the esterification bath was composed mainly of the spent liquor from the previous esterification, showed upon analysis 0 acetyl and propionyl contents varying not more than a few per cent from those obtained upon analysis of the ester produced in the original esterification. Of course, in carrying out a long series of esterifications the removal of moisture from the cotton by the acid anhydride, the losses due to the retention of liquid by the finished ester when it is removed from the liquid, and various other losses, must be considered, so that at intervals it may be necessary to add additional acid anhydride or additional non-solvent to the bath to replenish that which is lost in the continuous operation of the process, but such additions should not be construed as a departure from the scope of our invention.

If desired, processes according to the present invention may be carried out with an esterification bath which has been allowed to standuntil the ingredients have come to equilibrium, which procedure applied to baths for the dope or nonfibrous acetylation of cellulose: is old in the cellulose acetate art. Although the product resulting from a process employing such procedure is of excellent quality such procedure is of doubtful value in commercial practice due to the comparatively slow progress of the esterification.

Various well-known types of apparatus may be employed for the reaction vessel for the application of an esterification process in accordance with the present invention. For instance, the esterification maybe carried out in a tank with or without an agitating device such as a stirrer, and preferably with a water jacket to maintain uniformity of temperature; in a tumbling barrel, preferably jacketed; in a beater or hollander such as is employed in paper manufacture; in a bleaching engine of the type known in the paper industry as a Belmar; or in fact in almost any type of vessel which may be adapted for agitating and regulating the temperature of the mixture. Due to the soupiness or flowability of the reaction mass in the processes of the present invention,

much apparatus which would not even be suitable for an ordinary process for the preparation of an organic acid ester of cellulose would suggest itself as being suitable for a process in accordance with the present invention. Pumps for moving the liquors, centrifuges for separating the liquids from the cellulose ester etc. may be of the usual types employed in the prior art for similar operations.

Although cotton is the preferred cellulosic material for use in processes in accordance with the present invention, other materials such as sulfite, wood pulp or refined bagasse pulp are suitable for use therein.

It is to be understood that various non-solvents may be employed in processes carried out in accordance with our invention besides the petroleum hydrocarbon-chlorinated. hydrocarbon or high boiling aliphatic ether non-solvents employed in the examples. The use of the latter as a non-solvent is the invention of C. J. Malm and C. F. Fletcher and is disclosed and claimed in their application Serial No. 590,509. Some of the non-solvents which may be regarded as suitable depending on the ester to be prepared are benzene, carbon tetrachloride, or a petroleum distillate-aromatic hydrocarbon mixture, the use of which as a non-solvent in the fibrous esterification of cellulose is the invention of E. C. Yackel and W. O. Kenyon and is described in their Patent No. 2,036,382, which issued April 7, 1936.

In the preparation of mixed" organic acid esters of cellulose in accordance with the present invention, processes in which but one acid anhydride and a non-corresponding acid are used, such as for example acetic anhydride and propionic acid, propionic anhydride and acetic acid, butyric anhydride and acetic acid, or butyric acid and acetic anhydride, are also included; however, due to the ease of regulation of the proportions of the ingredients in the cases where the anhydrides and acids are employed in plurality in making the mixed" ester, the latter method is preferred. The present invention is adapted to the preparation of the simple esters of cellulose as is illustrated by Example VI. In addition to the acetate, the propionate or butyrate of cellulose may also be prepared by a process in accordance with our invention. Where the term "mixed" is employed with reference to esters or esterification baths it is to be understood as referring to esters containing a plurality of different acyl groups and to the baths employed in their preparation.

. In most cases it will be found desirable to anamate composition thereof from-"which the amount consumed of each anhydride employed and the increment of each fatty acid in the spent esterification bath may be determined, this information being of value in the application of a process in accordance with the present invention. The following method may be employed to analyze the ester where its content is to be determined by analysis;

A 0.500 gram sample of the thoroughly dried ester is weighed and placed in a 250 cc. Erlenmeyer flask. 20 cc. of 75% aqueous ethyl alcohol are added thereto, a rubber stopper loosely insertedin the fiask which is placed in an oven at a temperature of 57 C. for 30 minutes.- 20 cc. of 0.5 normal sodium hydroxide are then added, the flask tightly stoppered and placed in a 57 C. oven for 20 minutes after which it .is removed from the oven and allowed to stand at room temperature for 24-48 hrs. depending on the ester .being analyzed (24 hrs. for cellulose acetate, 36

The percentage of acyl groups in the ester in terms of acetyl groups (apparent acetyl") may be calculated by the following equation:

( apparent acetyl in which A is the number of cc. of 0.5-normal hydrochloric acid employed in the titration.

In the case where the ester is a mixed ester, it is necessary that the proportion of the various acyl groups be determined. The titrated sample is placed on the steam bath and evaporated just to dryness to drive off all the alcohol. 50 cc. of distilled water are added and the whole allowed to stand for hr. at room temperature with occasional shaking. The organic acids in the sample are then liberated from their sodium salts by adding an equivalent amount of 0.5 normal hydrochloric acid. This amount is determined from the initial titration. After the liberation of the organic acids in solution an additional 50 cc. of water are added and the sample shaken several times. The aqueous solution is then filtered from the regenerated cellulose on an ordinary funnel into a 250 cc. Erlenmeyer flask. The amounts of acetyl and propionyl or acetyl and butyryl groups are then determined by the partition method in which ethyl ether and water are employed in accordance with the method described by C. H. Werkman in the Iowa State Col-' lcge Journal of Science (4) July 1930, page 459. While it may not be necessary that the ester produced in carrying out the present invention be analyzed, such data would be of considerable value in estimating the proportions of ingredients 40 in the spent liquor for the purpose of adjustment of the amount of reuse. Another alternative which naturally suggests itself is the analysis of a sample of the spent esterification bath. The amounts of chemicals consumed or formed by the esterification may then be calculated by difference.

We claim as our invention: 1. A cyclic process for the preparation of an organic acid ester of cellulose in fibrous form by means of an acylation bath containing at least one lower fatty acid anhydride. at least one lower fatty acid, a sulfuric acid acylation catalyst and an inert organic non-solvent in an amount suflicient to make the bath non-solvent of the ester to be formed, which comprises reacting upon cellulose with a relatively large amount of that acylation bath whereby the anhydride and the catalyst present are depleted and fatty acid is accumulated, separating a large proportion of the spent liquor from the ester, segregating sufiicient of said separated spent liquor tosupply the lower fatty acid required for a succeeding acylation bath, adding to said segregated portion the kind and amount of the other acylation bath constituents required to reconvert said portion to a succeeding acylation bath of the same general character as the firstnamed acylation bath, and acylating further cellulose with the reconverted acylation bath. 70, 2. A cyclic process for the preparation of a mixed organic acid ester of cellulose in fibrous form, in an acylation bath containing at least one 7 lower fatty acid, the anhydrlde of at least one other lower fatty acid, a sulfuric acid acylation catalyst and an inert organic non-solvent in an amount suflicient to make the bath non-solvent of the ester to be formed, which comprises reacting upon cellulose with a relatively large amount of vthat acylation bath whereby the anhydride and the catalyst present are depleted and fatty acid is accumulated, separating a large proportion of the spent liquor from the ester, segregating sufiicient of said separated spent liquor to supply tthe lower fatty acid found in largest relative amount therein, which is required for a succeeding acylation bath of approximately the composition of the first named acylation bath, adding to said segregated portion the kind and amount of the other acylation bath constituents required to reconvert said portion to a succeeding acylation bath of approximately the composition of the first named acylation bath, and acylating further cellulose with the reconverted acylation bath.

'3. A cyclic process for the preparation of a cellulose acetate propionate in fibrous form by means of an acylation bath, containing at least one lower fatty acid anhydride and at least one lower fatty acid, supplying in the aggregate both acetyl and propionyl groups, a sulfuric acid acylation catalyst and an inert organic nonsolvent in an amount sufiicient to make the bath non-solvent of the ester to be formed, which comprises reacting upon cellulose with a relatively large amount of that acylation bath whereby the anhydride and the catalyst present are depleted and fatty acid is accumulated, separating a large proportion of the spent liquor from the ester, segregating sufllcient of said separated spent liquor to supply the lower fatty acid found in largest relative amount therein, which is required for a succeeding acylation bath of approximately the composition of the first named acylation bath, adding to said segregated portion the kind and amount of the other acylation bath constituents required to reconvert said portion to a succeeding acylation bath of approximately the composition of the first named acylation bath, and acylating further cellulose with the reconverted acylation bath.

4. A cyclic process for the preparation of cellulose acetate in fibrous form by means of an acylation bath containing acetic anhydride, acetic acid, an inert organic non-solvent and a sulfuric acid acylation catalyst, which comprises reacting upon cellulose with a relatively large amount of this acylation bath, which is nonsolvent of cellulose acetate, whereby the acetic anhydride and the catalyst present are depleted and acetic. acid is accumulated, separating a large proportion of the spent liquor from the ester, segregating sufficient of said separated spent liquor to supply the acetic acid required for a succeeding acylation bath, adding to said segregated portion the kind and amount of the other acylation bath constituents required to reconvert said portion to a succeeding acylation bath of approximately the composition of the first named acylation bath and then acylating further cellulose with the reconverted acylation bath, FREDERICK J. HOPKINSON.

CHARLES R. FORDYCE. 

